The US patent application publications 2018/0026525 and 2018/0024620 describe various configurations of PoDL systems that use combinations of CMCs and differential mode chokes (DMCs) to attenuate common mode noise while providing DC power to a twisted wire pair.
FIG. 1 is an example of one type of PoDL system described in the aforementioned publications. A PHY 10 outputs differential data and receives differential data via a conventional Media Dependent Interface (MDI) connector 12 coupled to wires 14 and 16 of a twisted wire pair. PHY 10 represents the physical layer in the OSI model and is a transceiver that typically includes signal conditioning and decoding circuitry for presenting bits to the next stage. The term PHY is a term of art and is defined by various IEEE standards, depending on the particular application. The PHY 10 is typically an integrated circuit. A digital processor (not shown) is coupled to the PHY 10 for processing the data.
The PHY 10 is connected to the MDI connector 12 via a CMC 18 and AC coupling capacitors C1 and C2. Termination resistors R1 and R2 and capacitors C3 and C4 are coupled to the wires 14 and 16, via the MDI connector 12, to eliminate reflections of the common mode noise on the twisted wire pair. The termination circuitry is generally designed to match the common mode impedance of the wire pair for maximum energy absorption and minimum reflectance while preserving the differential mode impedance presented by the transceiver. Other types of termination circuits can also be used.
The CMC 18 is an in-line transformer with two windings, where each winding is in series with a wire in the twisted wire pair. As shown by the dots on the CMC 18 windings, the windings have the same polarity, so the magnetic fields generated by a differential mode signal are substantially cancelled out. Thus, the CMC 18 presents little inductance or impedance to differential-mode currents. Common mode currents, such as ambient noise in the wire pair, however, see a high impedance due to the combined inductances of the windings.
The CMC 18 ideally eliminates or greatly attenuates common mode RF noise while providing no loss for the differential or DC voltage signals.
CMCs must present a low insertion loss to the differential data. However, CMCs have constraints which can impede their performance. Such constraints include inter-winding capacitance, DC resistance (DCR) of the windings, core loss, and limits on the current that can flow through the windings.
A DMC 20 is coupled between the MDI connector 12 and a DC power supply 22. The power supply 22 has a low output impedance as is characteristic of a voltage source. The DMC 20 has windings with opposite polarities (dots on opposite ends). The DMC 20 presents a high impedance to AC differential mode signals while it shunts the common mode signals to the power supply 22 due to its low impedance to common mode signals. Therefore, the combination of the CMC 18 and the DMC 20 can substantially remove AC common mode noise that has been coupled to the wire pair, especially at relatively low frequencies.
Since the DMC 20 improves the AC common mode rejection, the CMC 18 windings can be fabricated to have a lower inductance value, reducing the number of turns required, which affords benefits such as reducing the DCR of the CMC windings and reducing the parasitic capacitance. Alternatively, the CMC inductance can stay the same and the overall AC common mode rejection may be increased by the shunting action of the DMC 20.
However, there still exists a drawback with the configuration of FIG. 1. The DMC 20, in combination with the low impedance to ground of the power supply 22, will likely distort the common mode matching impedance provided by the RC termination circuitry, which was designed to eliminate reflections of the common mode noise signals. In other words, the termination circuitry, comprising the resistors R1 and R2 and capacitors C3 and C4, is designed to have an impedance that prevents reflections of the common mode signals while the low impedance by the DMC 20 to the AC common mode signals causes the resulting termination impedance (created by the RC circuitry and the DMC 20) to not be optimal to eliminate reflections of the AC common mode signals. Therefore, there may be some reflections of the AC common mode noise signals.
One way to avoid the DMC 20 adversely interfering with the operation of the termination circuitry for AC common mode noise signals is to place the DMC 20 on the PHY 10 side of the CMC 18, as shown in FIG. 2. In FIG. 2, the operation of the DMC 20 does not have a significant adverse effect on the RC termination circuitry, but now all the current transmitted by the power supply 22 must flow through the CMC 18, which requires that the CMC 18 be much more robust. Such a robust design requires a lower DC winding resistance and makes the CMC 18 more bulky and expensive compared to the CMC 18 in FIG. 1 for the same power requirements of the powered device (PD) connected to the other ends of the wires 14 and 16.
In FIGS. 1 and 2, the DMC 20 is designed to have a strong magnetic coupling between the windings. This rating is sometimes referred to in the DMC's data sheet as its “coupling coefficient” or the “coefficient of magnetic field coupling.” Such a high coupling coefficient is typically greater than 0.95. The absolute value of the coupling coefficient can be derived from the amount of leakage inductance. This leakage inductance translates to a frequency dependent impedance which can be compared to the transmitter's or cable's differential mode or common mode impedance to determine whether this is a low or high enough value. This strong magnetic coupling of the DMC windings is also referred to as the DMC windings being “tightly coupled”. By virtue of the tight coupling, such a DMC presents a very low impedance to common mode signals. On the other hand, a loosely coupled DMC has a larger leakage inductance, which presents a higher impedance to common mode signals, so is less effective in shunting common mode signals to ground via the power supply 22.
What is needed is a PoDL termination technique and DC voltage coupling technique that: 1) couples DC power to the wire pair without requiring the CMC to conduct the full current drawn by the powered device (PD); and 2) results in the DMC not presenting a low impedance to ground at the MDI connector for AC common mode noise signals so as not to adversely impact the effectiveness of an RC termination circuit that is designed to minimize reflections on the wire pair.